Commercially available wet process phosphoric acids are generally manufactured from either calcined or uncalcined phosphate rock. Calcining decomposes and drives off the organic matter in the rock, and the phosphoric acid product made by dissolving it, known as green acid, contains almost no suspended organic solids. When uncalcined rock is digested, considerable amounts of organic compounds are dissolved from the phosphate rock and remain as both soluble and insoluble impurities in the product acid, known as "black" or "brown" acid. These organic compounds in the acid are commonly referred to as humic acids or humic compounds.
The wet process phosphoric acid solution formed from uncalcined phosphate rocks generally contains about 600 grams/liter of H.sub.3 PO.sub.4, about 0.2 gram/liter of uranium, about 1 gram/liter of calcium, about 9 grams/liter of iron, about 28 grams/liter of sulfate and about 30 grams/liter of fluorine, The phosphoric acid solution also contains varying amounts of arsenic, magnesium and aluminum, and substantial amounts of humic acid impurities.
Uranium and other metals can be recovered from this type of commercial grade wet process phosphoric acid. Such recovery processes, directed primarily to uranium, are taught by Bailes and Long, in U.S. Pat. No. 2,859,092, and by Hurst and Crouse, in U.S. Pat. No. 3,711,591.
To make the metal recovery process viable, however, it is necessary to control sludge emulsion formation at the acidic solution-solvent interface in the solvent extraction mixer-settlers used in the metal recovery process. This sludge problem, caused primarily by the humic acids, was recognized by Hurst and Crouse in U.S. Pat. No. 3,711,591.
Reese et al., in U.S. Pat. No. 4,087,512, attempted to solve problems of uranium extraction emulsions and sludge formation, caused by humic acids in the wet process phosphoric acid feed, by a purification pretreatment with a kerosine type hydrocarbon at between 55.degree. C. to 70.degree. C. Such a process, however, introduces considerable complexity into the uranium recovery process. Also, such an approach has not been found particularly effective in removing a major portion of the humic acid over an extensive time period.
Simpler mechanical removal methods, such as pumping at the interface, after allowing sludge emulsions to form, are not particularly effective because large quantities of expensive solvent are also pumped out. This pumping method is also hampered by the non-Newtonian flow properties of the sludge emulsion, which cause it to deviate from normal pipe entrance behavior.
What is needed is a process to remove substantially all of the sludge emulsions which form at the acid-solvent phase interface during solvent extraction in metal recovery processes. The process must remove a substantial amount of the sludge, must be low in capital cost, and should result in low operating costs.